The method according to the invention is implemented in a metrology tool. The metrology tool comprises a measurement table which is movable in X-coordinate direction and in Y-coordinate direction, on which a mask is placed which exhibits the structures to be measured.
A metrology tool is well known in the state of the art. For instance, reference is made to the manuscript of the presentation “Pattern Placement Metrology for Mask Making” by Dr. Carola Bläsing. The presentation was given at the conference Semicon, Education Program in Geneva, on Mar. 31, 1998, and therein a metrology tool was described in detail. The setup of a metrology tool according to the state of the art is described below in the context of FIG. 1. A method and a measurement tool for determining the positions of structures on a substrate is disclosed in the German patent application DE 100 47 211 A1. For details of the mentioned determination of the positions of structures on a mask reference is therefore expressly made to this document.
The German patent DE 101 29 818 B4 relates to a method for reading data from a detection chip of an electronic camera in a metrology tool for determining the position of an edge of a structure on a substrate, with at least two digitizers reading from the detection chip, where to each digitizer are assigned individual pixels of the detection chip, and wherein for the extraction of characteristic measurement parameters the digitized data read from the digitizers are subject to a data reduction process. The aim of the method is to compare the image data of a large image-format camera including at a high reading rate at the computing power of essentially a fast personal computer and to extract characteristic measurement parameters if necessary. The method for reading data from the detection chip of a camera is characterized in that the comparison of the reduced digitized data of the different digitizers involves a correction function.
The German patent application DE 10 2008 002 770 A1 discloses a method for determining the positions of periodic structures on a substrate. In the method a metrology tool is used, comprising a measurement table which is movable in X-coordinate direction and in Y-coordinate direction. Based on measured images and/or measured intensity profiles and on calculated or adapted intensity profiles the positions of periodic structures on a substrate are determined.
The Japanese patent application JP 2232918 A1 discloses a position detector for the reduction of errors in the position measurement of alignment marks on a mask. The mask is rastered along a coordinate direction with a spot of light.
The US patent application US 2003/201404 A1 discloses a method for detecting the position of a structure (mark) on a substrate. The structure is a lattice structure with a small lattice constant. The light is incident on the structure under an angle. An attempt is made to determine the position of the structures from the recorded diffraction signal.
The U.S. Pat. No. 5,572,288 discloses an exposure system for imaging the structures present on masks onto the surface of a wafer. An optical alignment system is provided which determines the alignment from marks.
A metrology tool is furthermore known from a plurality of patent applications, like for example DE 198 58 428, DE 101 06 699, or DE 10 2004 023 739. All documents of the state of the art listed here describe a metrology tool that allows to measure structures on a substrate. The substrate is placed on a measurement table which is movable in X-coordinate and in Y-coordinate direction. The metrology tool is constructed in such a way that the positions of structures or of edges of structures, respectively, are determined with an objective. For determining the positions of the structures or of their edges, respectively, the position of the measurement table needs to be determined with at least one interferometer. Finally the position of the edge is determined with respect to a coordinate system of the metrology tool.
Up to now all scanning algorithms of position metrology rested on the assumption that the object (structure) to be measured, at least within the area of the measurement window, does not change in the direction orthogonal to the measurement direction. In the case of the documents mentioned above (JP 2232918; U.S. Pat. No. 5,572,288; US2003/201404) a spot of light is moved over the edges of the structures along the measurement direction. In the documents DE 100 47 211 B4, DE 101 29 818 B4 and DE 10 2008 002 770 A1 the image information is averaged in the direction orthogonal to the measurement direction. In both cases no information regarding changes of the structures along a direction orthogonal to the measurement direction explicitly enters the scanning algorithm.
More recent mask technologies use increasingly smaller and finer structured objects (miniaturisation, OPC, double patterning). These are imaged with a diffuse edge due to the limited resolution of real optical systems. Therefore frequently one has to deal with structures where no edge sections which don't change in a direction orthogonal to the measurement direction can be found, so that known algorithms record the mean value of a curved edge. These results can be interpreted only with difficulty and indirectly; also often the result depends sensitively on the position of the measurement window.